Photo Finish Update: 1. Preamable


Photo Finish equipment has been used for some time in sports such Athletics, Cycling, Horse Racing and Skiing. Of late have working towards a complete software system for adjudicating sporting races in terms of times and finish order. My interest is largely on the Athletics domain. Athletics events that are sanctioned by athletic associations use approved equipment that implements. The video camera is aligned with the track’s finish line such that for any video frame, that line is the middle vertical line of pixels. Note that whilst athletics tracks are typically a reddish or blue color, a distinctive white line across the finish line as paint or tape means that except for finishing competitors, the middle vertical line is white.

A Sample Video Finish Image

Comment. Note that the trailing legs appear horizontally stretched as these are moving backwards relative to the athlete when crossing the finish line and so take longer to cross the finish line.

Some years ago I developed some software that took a video and stitched an image into a linear image file taking middle vertical line of pixels from each video frame using a native software API. Time to revisit that.

Photo Finish

Being “approved” means that records and individuals’ qualifications for championships are recognized when timed using this equipment. The equipment includes a very fast video camera, computer and relevant software for recording and analyzing the finish line video in a timely manner, plus interface equipment to get the video data onto the computer’s storage

Phototiming processing involves taking the middle vertical line of pixels from each video frame and sequentially stitching them into an image such that the horizontal axis represents time. An athlete’s finish time and relative position is then taken from when their shoulder is first visible in one of those vertical image slices. A fast finishing athlete will appear as a borrower blob in the image, than a slower one. For lane based events, the vertical position of their image is used to determine their lane.

Phototiming is typically measured to 1000th of a second and the video FPS needs to be faster that that. Altogether that makes a complete Photo Finish system not a cheap purchase. For lower rung competitions it is then an overkill. There are various alternative timing systems in use for athletics such as timing gates and in ground or track inside sensors where athletes carry a a unique but complementary sensor. Also, for distance events, a loop in ground or about the finish line of an event, is activated by a passive sensor attached to the athlete.

Guidlines:

  • Ensure the camera is fixed and aligned perfectly with the finish line.
  • Use consistent lighting to avoid motion blur.
  • If possible, capture at higher frame rates for better timing accuracy.

Home Brew Photo Finish

A system is being developed that uses a mobile phone to capture the video of an event finish line with processing performed by desktop app after transference of the video to the desktop. A custom Android app has been developed for video capture encompassing various event start tracking methods. Remote control of the phone has been developed so that it can be mounted and unmoved whilst capturing the video. The image stitching can be performed on the phone but is better performed on the desktop where actual video frames can be viewed if required. Software has been developed to reliably transfer the video to the desktop. The video has meta information about start annotation method used and in one case the wall clock event start time. The desktop app is then able to process the video into a “stitched” image with knowledge of the start time which may or may not be the video start time. One option is that the start information is embedded in the video for discernment, such as an auditory “bang” or visual flash.

System Status

The following have been developed and tested:

  • Android phone app to capture the video with various video start methods and gun time annotation (these may or may not be the same).
  • A remote control for controlling the phone to signal the video start and the optional gun time; over Bluetooth.
    • An Android app for performing the image stitching
  • TCP desktop service Windows WPF app to receive videos plus an Android client app to send it.
  • A desktop WPF app for performing the video stitching, marking on the image of the finish of athletes from their torso and subsequent calculating timing wrt the event start time. Includes inspection of video frames corresponding to the vertical line in the image for clarification when required.

The next steps are to:

  • Integrate the TCP desktop app into the main app.
  • Add video download triggering via the remote control or automate it.

Start Options

There is the gun time and video start time which may may not be the same. Starting the video at gun time is probably the least accurate.

  • Start the video when the event starts.
  • Auditory from gun sound recorded with the video
    • Video audio is processed by the desktop app.
  • Visual from a flash recorded as part of the video capture
    • Video is inspected or image processing is used by the desktop.
  • Separate gun fire wall clock time recorded and added to video metadata, which when processed on desktop is compared to video wall clock star time.
    • Note that the gun time can be before or after the video start.

Camera Timing Resolution

Phone video frame rates are 30 fps or above. My phone does 30 and 60 fps. Other phones have higher frame rates. The framerate limits the timing accuracy. For example, 30 fps means a new frame every 32 milliseconds. 60 fps means one every 16 milliseconds. In these cases, timing resolution would be these times, or perhaps twice those frame periods.

Commercial photo finish cameras are much faster. The cameras should be scanning at least 1000 lines per second (preferably 2000), and the precision should be at least 1000th of a second (or finer).
Ref: IAAF Photo Finish Guidelines PDF

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